1. Field of Invention
The present invention relates to optical networks, such as MAN (metropolitan area networks), SAN (storage area networks) and access optical networks. In particular, the present invention relates to a synchronization scheme for dynamic optical networks, which utilize ultra-fast tunable lasers for direct all-optical routing.
2. Description of Related Art
Fiber-optic infrastructure is a vital part of today""s rapidly changing worldwide networks. The drive for interconnectivity as well as the exponential growth in data traffic, as a result of new applications, requires the adoption of new optical solutions. Carriers and service providers are looking to increase their revenue by delivering new services such as storage area networks and IP based services to their customers. New technologies that can leverage the existing network, as well as increase the economic viability of new network applications, are needed. New market opportunities and the recent advances in optical technologies (such as wavelength division multiplexing, tunable lasers and high speed optical/electronic components) have yielded new developments in the optical networks area.
Traditionally, optical networks were used mainly in long-haul area networks. Today, however, new optical networks are being introduced in the regional, metropolitan and the access area networks. The long-haul area networks use a fiber-optic infrastructure to create large data pipes between two distanced points. Contrastingly, the new optical networks are facing different demands.
Optical networks in the regional, metropolitan and the access area networks require a sustained, high bandwidth, while maintaining mesh connectivity and supporting multiple services and multiple classes of service. For example, metropolitan area networks can transport voice traffic, SAN traffic and other IP traffic. Voice traffic demands low bandwidth, with a guaranteed bandwidth, while SAN traffic is delay sensitive and is burst traffic. An IP traffic class of service is application dependent. Optical network are required to aggregate multiple types of data and transport while keeping the required quality of service.
Communication networks can be divided into two general types: circuit-switched networks (typically used for telephony traffic) and packet-switched networks (typically used for data traffic). Circuit-switched networks (like SONET or SDH) are networks wherein connections between nodes are fixed, whether data are crossing the connection or not. Each connection in a circuit-switched network has a constant-bandwidth. Packet-switched networks, on the other hand, are connectionless networks, wherein data is transmitted in a burst mode. The benefit of packet-switched networks is that bandwidth is used more optimally. However the connectionless networks lack the ability to reserve bandwidth, and support a hard quality of service. Moreover the statistic aggregation of packets can create an overload situation, in which packets may suffer large delays or even data loss.
All optical networks are basically packet-switched networks, in which routing packets from a source node to a destination node is done optically, without the need for optical-electrical conversions outside the source and destination nodes. A sub-group of the all-optical networks is the all-optical multi-ring networks. Optical multi-ring networks are based on a fiber ring topology, in which the fiber-ring is a shared optical medium. Nodes, located around the fiber-ring, are equipped with optical receiver, fixed to a unique wavelength, and with an ultra-fast tunable transmitter. In a multi-ring optical network each wavelength is associated with a specific node. Transmitting packets to destination node is done by a tunable-laser tuned to the destination wavelength. Logically the network is a multi-ring topology, which allows any node to address any other node simply by changing its transmitter wavelength to the target""s receiver wavelength without electrical routing.
As in other shared media topologies, in the packet switched multi-ring topology, the problem of collisions when accessing the fiber must be addressed. There are two main approaches to resolve packets collision problems: 1) an Ethernet-like scheme where carrier detection is applied or 2) a synchronous system where collision can""t happen since each participant (node) has a reserved time when it can use the media. By giving each node a required time or time frame in which it can use the media, a synchronizing scheme between all the nodes needs to be implemented. In the case of optical slotted ring dynamic networks, packets can be transmitted to the fiber in dedicated time slot boundaries. The optical media poses special problems that need to be overcome with respect to the slot synchronization.
Furthermore, since packets can be transmitted from any one node to any other node on the fiber ring, frequency, phase and the position of the payload inside the time slot cannot be guaranteed. Synchronizing on the signal frequency and phase, and recognizing the payload should be done within a fraction time of the packet duration and calls for fast synchronization solutions.
Thus, there is a need for a synchronization scheme that addresses the problems discussed above for use in dynamic optical networks. There is also a need for a process and system that overcomes the problem of time slot synchronization between the nodes on the fiber ring so it is possible for each node to access the fiber without collisions. Further more, in the context of this synchronization method, there is also a need to define a method to recover the packet""s data and clock in an ultra-fast recovery time.
It is an object of this invention to overcome the drawbacks of the above-described conventional network devices and methods. The present invention provides for a new synchronization method for an optical slotted ring dynamic network. With this approach, nodes that send packets to the same destination node must access the fiber at a designated time-slot. The synchronizing signal is sent from a master node to the other nodes. The present invention also provides for a burst mode receiver used to receive and process an optical signal.
According to one aspect of this invention, a method for communicating data over a network having a plurality of nodes thereupon is disclosed. A time slot clock signal is transmitted from one node of the plurality of nodes to other nodes of the plurality of nodes. After each of the other nodes of the plurality of nodes receives the time slot clock signal and a return signal is received, the time slot clock signal is recalculated to achieve an integer number of slots on the network. The recalculated time slot clock signal is transmitted from the one node of the plurality of nodes to the other nodes of the plurality of nodes.
Additionally, the method may be applicable to an optical fiber ring network and the time slot clock signal may be transmitted from the one node to the other nodes of the optical fiber ring network. Also, the method may include waiting for a last of the other nodes of the optical fiber ring network to send a signal to the one node of the optical fiber ring network. In addition, the method may be performed periodically to maintain the integer number of slots on the network.
In addition, the step of recalculating the time slot signal to achieve an integer number of slots on the network can include changing a duration of the time slot used in the time slot clock signal and/or changing the optical length of the optical fiber ring network. The change in the optical length is used to make a rough adjustment to achieve the integer number of slots and the change in the duration of the time slot is used to make a fine adjustment to achieve the integer number of slots. The optical length of the optical fiber ring network may be changed by adjusting an optical delay line.
Also in the method embodiments of the present invention, a system bit clock signal may be transmitted that includes the time slot clock signal contained therein. Also, a packet may be transmitted from a particular node of the plurality of nodes to another node of the plurality of nodes within one of the integer number of slots on the network and the packet can also include inserted guard times before and after the packet within the one of the integer number of slots. Additionally, the packet may include a preamble, a barker and a packet payload, where the barker is used by a receiver of the another node of the plurality of nodes to extract the packet payload.
The method may also include waiting for the time slot clock signal to pass through a coupler connected with each of the other nodes of the plurality of nodes and to arrive back at the one node. Also, the time slot clock signal may be received and retransmitted by each of the other nodes of the plurality of nodes, before it arrives back at the one node. Signal data may be added or dropped from the time slot clock signal when the time slot clock signal is received and retransmitted by each of the other nodes. Also, one node of the plurality of nodes may be a master node that transmits and maintains a system bit clock using a broadcast wavelength.
In another aspect of the invention, a communications node for an optical fiber network is disclosed. The communication node includes a fixed wavelength receiver for receiving optical data, a tunable wavelength transmitter for transmitting optical data to destination nodes at a plurality of destination wavelengths and a media access controller that determines a slot clock based on a system clock signal received by the fixed wavelength receiver time slots. The tunable wavelength transmitter uses the slot clock to determine a slot in which the optical data is to be transmitted. The fixed wavelength receiver may include a phase-lock-loop. The fixed wavelength receiver may be a burst mode receiver and may include a phase shifter to provide a phase-shifted system clock. Also, fixed wavelength receiver may have at least two direct digital synthesizers receiving the same control signal from a controller, used to shift the phase of the receiving optical data. The fixed wavelength receiver can also be capable of creating a phase controlled high bitrate clock.
In another aspect of the invention, a burst mode receiver is disclosed. The burst mode receiver includes a phase shifter, receiving a system clock and producing a phase-shifted clock, a controller in communication with and controlling the phase shifter and a sample unit, receiving a phase-shifted clock and producing sampled data. The phase shifter may include at least two direct digital synthesizers receiving the same control signal from a controller, used to shift the phase of the receiving optical data. Additionally, the burst mode receiver may be capable of creating a phase controlled high bitrate clock.
These and other objects of the present invention will be described in or be apparent from the following description of the preferred embodiments.